Means for generating time modulated electrical pulses



1949 M. M. LEVY 2,492,161

MEAKS FQR GENERATING TIME MODULATED ELECTRICAL PULSES Filed May 30, 19442 Sheets-Sheet 1 DEM Y NETWORK 6W8 FILLED lNVENTOR MflU/P/CE M0155 LEV)ATTORNEY Dec. 27, 1949 M. M. LEVY 2,492,161

MEANS FOR GENERATING TIE "ODULATED ELECTRICAL PULSES 2 Sheets-Sheet 2Filed May 30, 1944 4' Tee INVENTOR MAUF/Cf Mal/$515 ATTORNEY PatentedDec. 27, 1949 MEANS FOR. GENERATING TIME MODU- LATED ELECTRICAL PULSESMaurice Moise Levy, London, England, assignor,

by mesne assignments, to International Standard Electric Corporation,New York, N. corporation of Delaware Application May 30, 1944, SerialNo. 538,061 In Great Britain June 1, 1943 Claims. (01. 332-14) Thepresent invention relates to arrangements for deriving a phase-modulatedtrain of short electrical pulses from a modulated carrier wave.

A train of pulses is phase-modulated when the time interval betweensuccessive pulses is varied according to the modulating signal, theduration or form of the pulses remaining unaltered.

The principal object of the invention is to provide a simple method ofobtaining a phasemodulated train of pulses directly from a modulatedcarrier wave without an intermediate demodulation. The arrangements aresubstantially the same whether the carrier wave is amplitude-,frequency-, or phase-modulated.

According to the invention there is provided an arrangement forobtaining a phase-modulated train of short electrical pulses, comprisinga source of a signal modulated sinusoidal carrier wave, means forgenerating a train of pulses, and means for applying the carrier wave tosynchronise the generating means in such a manner that the time spacingof the generated pulses varies in accordance with the modulating signal.

According to another aspect, the invention,

consists in an arrangement for phase-modulating a train ofshortelectrical pulses in accordance with a signal, comprising means forgenerating a sinusoidal carrier wave, means for modulatingthe wave bythe signal, means for generating a train of pulses, and means forapplying the modulated carrier wave to control the times of emission ofthe pulses in such a manner that the interval between successive pulsesvaries in accordance with the signal amplitude.

According to a further aspect, the invention provides an arangement forderiving a phasemodulated train of short electrical pulses from anamplitude-modulatedsinusoidal carrier wave comprising pulse generatingmeans connected to the source of the carrier wave and means for causingthe carrier wave to synchronise the pulses in such a manner that thetime at which any pulse is emitted is a function of the instantaneousamplitude of the modulating signal at that time.

The invention will be described with reference to the accompanyingdrawings in which:

Fig. l is a diagram used to explain a preferred synchronisingarrangement employed in the present invention;

Fig. 2 shows a schematic circuit diagram of a pulse generatingarrangement embodying the present invention;

Fig. 3 gives diagrams used in explaining the action of the system ofFig. 2;

Fig. 4 is a diagram used to explain the action of the system of Fig. 5;

Fig. 5 shows a schematic circuit diagram of a multivibrator synchronisedaccording to a preferred arrangement; and

Fig. 6 is a diagram used to explain the action of Fig. 5.

The invention employs pulse generating arrangements which are adapted tobe synchronised by a controlling wave, preferably, though notnecessarily, by the methods described in the specification of copendingapplication Ser. No. 526,605, filed March 15, 1944, now Patent No.2,462,109, issued February 22, 1949.

The invention described in the above-quoted specification is concernedwith pulse generators and improving the stabflity of the synchronisationon a submultiple of the controlling frequency. Fig. 1 shows the methodadopted.

Short duration unidirectional pulses are derived from saw-tooth wavesand are delayed and mixed with 'a control wave. The sign of the pulsesis chosen so that they momentarily reduce the negative bias applied tothe control grid of the pulse generator valve. The effect of thestriking voltage curve is shown in Fig. 1. It will be seen that thepulses depress every tenth loop of the control wave so that the upwardslope of the saw-tooth wave can make contact therewith. If the amplitudeof the synchronising pulses is chosen so that contact is made not farfrom the centre of the flank of the controlling wave as shown, thenconsiderable variation of the operating voltage or amplitude of thecontrolling wave is allowable before control is lost. and the adjacentloop of the control wave is out of the way and therefore cannotintercept the saw-tooth curve.

As will be explained later, the pulse which causes the discharge of thecondenser C1 in Fig. 2. thereby producing the fiy-back stroke, isderived after a definite delay from the preceding fly-back stroke of thesaw-tooth wave. So long as the delay is approximately correct, thedesired result will be obtained, and errors in the time spacing of thepulses do not accumulate.

Fig. 2 shows one arrangement in which the synchronising is carried outby the method described with reference to Fig. 1. The circuit forproducing the saw-tooth waves comprises a gasfllled valve V1 arranged asa relaxation oscillator periodically discharging condenser C1 which ischarged via resistor R1 by the high tension source 3 HT. The valve V: isarranged as a simpie pulse generator which is excited by the saw-toothwaves obtained through condenser C: from a tapping on the resistance R1.The control grid of V2 is connected to the cathode through two seriesresistances R2 and R3 and a battery or vother source B: adapted to applya positive potential to the control grid. The resistance R; should be arelatively high resistance adapted to ensure that the positive gridpotential is small.

The point on R1 to which the condenser C: is connected should be chosenso that the sudden reduction in the voltage applied to the control gridof V: which occurs at the fly back stroke of the saw-tooth waveissuflicient to block the valve. The condenser C: becomes charged at thesame time, and discharges through the resistance R: and the lowerportion R of R1. If the time constant C:.(R:+Rs) is small compared withthe time constant C1.R1 of the relaxation circuit, then the potential ofthe grid of V: will quickly rise until the valve is unblocked and anodecurrent begins again to flow. Thus the anode potential of V: suddenlyrises to a maximum on the occurrence of the fiy-back stroke, and thenremains constant for a short period determined by the time constantC:.(Rz+Ra) and then falls to the original value again, producing verynearly rectangular pulses across the anode resistance R4. In Fig; 6curve a shows the sawtooth waves and b the voltage pulses obtainedacross the resistance R4. These pulses are applied through a blockingcondenser C3 to the input terminals of a delay network DN, the outputterminals of which 'are terminated with a resistance R5. The upperterminal of R5 is connected by the conductor a: back to the control gridof the valve V1 through the biassing battery B: and the secondarywinding of the transformer T.

Delayed pulses will be obtained at the output of DN as shown in Fig. 3,curve 0. The nth pulse of c is delayed with respect to the nth pulse ofb by a time T1 which is very slightly shorter than the desired period Tof the saw-tooth waves, so that the (n+1) th fiy-back stroke of thesaw-tooth wave occurs approximately at the centre of the nth delayedpulse. The delay T1 can be obtained by appropriate design of the delaynetwork DN.

The duration of the pulses b and 0 can be controlled by adjusting thetime constant which may be done by varying any or all of the componentsC2, R: and Rs.

It should be mentioned that the battery B3 is not essential and may beomitted. The circuit will'operate in this case in very nearly the sameway; the grid of V; will, however, be slightly negative instead ofslightly positive in the intervals between the pulses.

One preferred form of the delay network DN consists of an artificial.non-dissipative transmission line which may be made up of a number oflow-pass filter sections, comprising series inductances and shuntcondensers, adjacent in ductances being preferably mutually coupled. Thesections should be chosento be sufliciently short so that a high cut-oilfrequency is obtained, in order not appreciably to deform the pulses.Such an artificial line is very convenient because the delay issubstantially independent of frequency over a wide range, and if meansis provided for tapping oil at any section of the filter, a delayadjustable over a wide range may be obtained. If such a delay network isused, the resistances R4 and R5 should be adjusted so that the networkis terminated at both ends by its characteristic impedance, in order toavoid reflections of the pulses at the terminals.

The pulses generated by the methods already described will be slightlytrapezoidal in form on account of the slope of the trailing edgesresulting from the discharge curve of the condenser C2. By a slightmodification of the arrangement of Fig. 2, truly rectangular pulses canbe obtained. In this modification the conductor in is removed, and theconnection back to the control grid of V1 is taken instead through theconnection 1/ to an adjustable tap t at some intermediate point of thedelay network DN. The resistance R5 should also be short-circuited. Thegrid battery or other source B: should preferably in this case bias thegrid negatively.

The time constant C2(R2+Rs) should be adjusted to be rather larger thanbefore, so that the duration of the pulses generated by the valve V: isnot much smaller than the period of the sawtooth wave. Such pulses areshown at d in Fig. 3. These pulses are transmitted through the delaynetwork DN and reach the tap t after a time T1 slightly less than theperiod of the saw-tooth wave, as shown at e. The pulses continue to theshort circuited end, where they are reflected with an inversion andreturn to the tap t after a further. short period T2, as shown at Theycontinue back to the input end of the delay network where they areabsorbed without reflection, the network being properly terminated by R4as already explained.

Curve 9 of Fig. 3 shows the resultant eifect at the tap t. Shortrectangular upwardly directed pulses are obtained by the superpositionof the two series of pulses e and 1, each pulse being preceded by adownwardly directed Vshaped pulse. This latter pulse could be removed byamplitude limitation or in any other way if desired, but its presence isnot harmful, since the effect will be only to raise momentarily thestriking voltage of the valve V1 at a time when it is not required tostrike.

It will be evident from the above explanation that the duration T2desired for the pulses is obtained by arranging the tap t at a distancefrom the short-circuited end corresponding to T2/2. The delay period T1which will be approximately equal to TT2/2 gives the distance betweenthe input terminals of the network and the tap t. In practice it will befound convenient to provide an adjustable short-circuiting switch (notshown) by which the network may he short-circuited at any desiredintermediate point. By this means both the times T1 and T2 can easily beselected for a variety Of conditions.

In Fig. 2 an output terminal 6 is provided from which the synchronisedpulses may be obtained.

The same principles may also be applied to synchronising a multivibratorcircuit on a submultiple of the controlling frequency. In Fig. 5

is shown one well known form of a multivibrator comprising two valves Vaand V4 having their control grids and anodes cross-connected throughcondenser-resistance circuits C1, R7 and Cs, Ra- The resistances P areappropriate anode current supply resistances, and the grids arenegatively biassed by suitable means represented by batteries B5 and B6.The properties of this circuit are explained in detail in thespecification of application Ser. No. 445,510 filed June 2, 1942, now

Patent No. 2,436,808,. but its action will be briefly described here.

The valves Va and V4 are alternately cut-oi! and saturated for periodsdetermined by the time constants C'aR-z and Calla. Thus the grid voltageof V3 remains below the cut-off point for a short period, determined bythe time constant (21.121. during which the anode voltage remainssubstantially constant at a maximum value. During this period the gridvoltage of V4 is above the saturation point, so that the anode voltageremains substantially constant at a minimum value. At the end of thisperiod the grid voltage of Va reaches the cut-off point, and thecondition suddenly changes over so that V: is now saturated and the gridvoltage of V4 is taken below the cut-oil point. This condition persistsfor a longer period determined by the time constant CaRa. during whichthe grid voltage of V4 rises to the cut-ofl point, on

which the condition suddenly changes back again and the cycle isrepeated. Thus it will be seen that positive short duration voltagepulses are generated at the anode of V: and inverted pulses of similarduration are generated at the anode of V4.

Fig. 4 shows the time variation of the grid voltage of the valve V4. Inthis flgure the dotted line Vc indicates the cut-off voltage, and thedotted line Va the saturation voltage.

The emission of a pulse is determined when the grid voltage reaches theline Vc, on which it suddenl rises above the line Va as shown at a. Itthen falls, following the curve b which is determined by the dischargeof the condenser 01, and then drops suddenly below the cut of! at c,this taking place at the moment of unblocking of the valve Vs. The gridvoltage then rises rather slowly, following the curve d determined bythe discharge of the condenser Ca, until the line Vc is again reached.when the process is repeated. In order to obtain rectangular pulses atthe anode of V4 it is necessary to arrange that the curve b does notreach the line Va before the circuit changes over.

The frequency of repetition of the pulses is determined by the curve atwhich in turn depends on the time constant CaRq. It will be evident thatthe time at which the curve It reaches the line Vc will be verysensitive to small changes in Vc or in the values of Ca and Rs, so thatthe arrangement is found to be very unstable and irregular in operation.

The arrangement of Fig. 5 may be synchronised on the frequency of acontrolling sinusoidal wave, or on a s-ubmultiple, supplying the wave inseries with the resistance Rs, so that the vertical strokes a (Fig. 4)occur at times determined substantially by the controlling wave, thecurve d merely deciding which submultiple is to be employed. As in thecase of Fig. 2, the synchronising becomes rather unstable forsubmultiples of high order, and it is therefore preferable to employsynchronising pulses in the manner shown inFig. 5.

Connected in series with the resistance Ra (which is the one which isconcerned in defining the frequency of repetition) is the secondarywinding of a transformer T through which the controlling wave isapplied, and a resistance R0 shunted by two delay networks DNI and DNZ.These networks are used for generating synchronising pulses whichoperate similarly to those previously described. Ignoring these pulsesfor the moment, it will be understood that the synchmnising wave will besuperposed as "ripples" a pulse, it will be raised up out of the way ofthe preceding ripples and the arrangement becomes stable. This can beseen from Fig. 6 which shows to an enlarged scale part of Fig. 4 whenthe synchronising wave and pulse are supplied. In Fig. 6, the full linerepresents the actual variation of the grid voltage.

The negative grid bias voltage produced by Be is first of all increased,so that the dotted discharge curve a would occupy the position shown inthe absence of the synchronising waves. The leading edge I of thesynchronising pulse is timed to occur slightly before the time when thegenerated pulse is emitted, and its duration should be about equal toone period of the synchronising wave. One loop of the wave is thusraised up so that it cuts the line Vc causing the generated pulse to beemitted. The leading edge of this pulse is shown at a.

The synchronising pulse for the (n+1)th pulse of Fig. 4 is obtained fromthe nth pulse by reflection in the delay network DNI whose outputterminals are left open as shown. In this way the nth pulse which isgenerated across the resistance R9, is reflected back without aninversion, as desired. The total delay suffered by the reflected pulsein DNI should be a little less than the repetition period T desired forthe generated pulses. The network BN2 has its output terminalsshort-circuited in order to obtain an inverted reflected pulse delayedslightly longer than the pulse reflected in DNI so that the combinationof the two reflected pulses may produce a short synchronising pulse in amanner similar to that describedwith reference to the curves d to a ofFig. 3. If the duration of the synchronising pulse is to be T2, thennetwork BN2 should delay the reflected pulse-by a total time which isgreater by T: than the total delay in DNI. Alternatively, the delaynetwork BN2 may be arranged to delay the reflected pulse by a total timeequal to T2. Thus, when a pulse appears across the resistance Ro. thenetwork DN! produces a synchronising pulse having the required durationT: which pulse is then delayed by the desired amount in the network DNIin the manner explained.

, It may be added that the network DN2 is not essential and can beomitted, in which case the synchronising pulse will have the sameduration as the emitted pulse. This will operate satisfactorily sincethe synchronising pulse will have iisappeared before the emitted pulseis due to disappear.

It is to be noted that the resistance R0 should be chosen so that thenetworks are correctly terminated, in order to avoid any furtherreflections of the pulses at the input terminals of the networks.

Reference will again be made to Fig. 2. Let an amplitude modulatedcarrier wave be applied as a controlling wave to the terminals I and 2instead of the usual simple sinusoidal wave. Thea effect will be that inFig. l the amplitude of the wave 4 will vary. When the amplitudeincreases the ny-back stroke will occur a little earlier than usual, andwhen it decreases it will occur a little later.

The use or synchronising pulses has another advantage. From Fig- 1 itcan be seen that the synchronising point can be arranged anywhere on thedescending flank of the loop, by suitably choosing the amplitude of thesynchronising pulse. Of course, if the synchronising point be chosenexactly at the centre of the flank of the loop, the phase modulation ofthe generated pulses will be zero since this point is not shifted by theamplitude modulation, but if any other point on the substantiallystraight part of the flank be chosen, not too far from the centre, thephase modulation of the pulses will not sufl'er appreciable distortion.

It will be evident from what has been said that the multivibratorcircuit of Fig. 5 may also be made to generate phase-modulated pulses bysupplying an amplitude-modulated carrier wave, the modulated carrierwave being supplied to the terminals I and 2, the amplitude of thesynchronising pulse being so chosen that the flank of the synchronisingloop of the control wave intersects the line Vc at some point other thanthe centre.

In the above explanation it has been assumed that the controlling waveis amplitude-modulated. It will be obvious that the arrangement willalso operate if the controlling wave is phaseor frequency-modulated. Inthat case, the amplitude remains constant but the loops are shiftedsideways by the modulation, some coming sooner and others later thanusual. It will be evident that the saw-tooth waves and the pulsesderived from them will be phase modulated accordingly. The preferredarrangement shown in Fig. 1 in which synchronising pulses are used alsooperates satisfactorily, since although the synchronising pulses willnot now generally register accurately with the synchronising loops, thisis not of much importance as only approximate registration is necessary.Similar remarks apply to the arrangement of Fig. 5.

The principles on which the invention is based have been explained withreference to certain specified circuit arrangements which, however, donot represent every possible arrangement. In essentials the inventioncomprises a source of a modulated carrier wave connected to a pulsegenerator so that the carrier wave controls or synchronises'thegenerated pulses, so that they are phase-modulated in accordance withthe original modulating al.

The source of the carrier wave may be, for example, a transmissionchannel of any kind conveying the modulated wave from a distant point inthe usual way. It may, however, be a convenient modulating arrangementcomprising, for example, a modulator supplied simultaneously with asignal wave and a locally generated carrier wave, in which case thearrangement of the invention provides a means for phase-modulating apulse train with the signal wave.

Preferably, synchronising pulses are combined with the modulated carrierwave in order to improve e stability of the arrangement, and to enablethe conditions to be chosen so that the modulation of the pulses issubstantially free from distortion, as explained.

What is claimed is:

1. An arrangement for phase-modulating a train of short electricalpulses in accordance with a signal, comprising means for generating asinusoidal carrier wave, means for amplitude modulating the wave by thesignal, means for generating a train of pulses, means for synchronizingthe generation of the train of pulses on a submultiple of the carrierfrequency means for applying the modulated carrier wave to saidgenerating means to control the times of emission of the pulses in sucha manner that the interval between successive pulses varies inaccordance with the signal amplitude, means for deriving synchronizingpulses from the pulse generating means and means for applying thesynchronizing pulses in conjunction with the carrier wave to the pulsegenerating means for synchronizing the pulse train 2. An arrangementaccording to claim 1 in which the pulse generating means comprises arelaxation oscillator for generating waves of a i saw-tooth type andmeans for deriving substantially rectangular pulses from the saw-toothwaves.

3. An arrangement according to claim 1 in which the pulse generatingmeans comprises a multivibrator adapted to generate substantiallyrectangular pulses directly.

4. An arrangement according to claim 1 including a delay network forderiving the synchronising pulses from the pulse generating means.

5. An arrangement according to claim 1 in which the pulse generatingmeans includes a relaxation oscillator for generating saw-tooth waves,said oscillator comprising a gas filled triode arranged periodically todischarge a condenser, means to derive from the saw-tooth wavesthemselves a train of uni-directional pulses repeated at a sub-multipleof said carrier wave frequency, a delay network for delaying the saidsynchronizing pulses by a time interval slightly shorter than the periodof the saw-tooth wave, and means to apply the pulses from the delaynetwork in conjunction with the modulated carrier wave to the controlgrid of said triode.

MAURICE MOISE LEVY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS 2,413,182 Hollingsworth et a1. Dec. 24, 1946

